Re: autofs: add autofs_parse_fd()

[Ian Kent]

On 23/10/23 21:57, Anders Roxell wrote:
> On Mon, 23 Oct 2023 at 09:35, Ian Kent <raven@xxxxxxxxxx> wrote:
> > On 23/10/23 08:48, Ian Kent wrote:
> > > On 20/10/23 21:09, Ian Kent wrote:
> > > > On 20/10/23 19:23, Arnd Bergmann wrote:
> > > > > On Fri, Oct 20, 2023, at 12:45, Dan Carpenter wrote:
> > > > > > On Fri, Oct 20, 2023 at 11:55:57AM +0200, Anders Roxell wrote:
> > > > > > > On Fri, 20 Oct 2023 at 08:37, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > > > > > On Thu, Oct 19, 2023, at 17:27, Naresh Kamboju wrote:
> > > > > > > > > The qemu-x86_64 and x86_64 booting with 64bit kernel and 32bit
> > > > > > > > > rootfs we call
> > > > > > > > > it as compat mode boot testing. Recently it started to failed to
> > > > > > > > > get login
> > > > > > > > > prompt.
> > > > > > > > >
> > > > > > > > > We have not seen any kernel crash logs.
> > > > > > > > >
> > > > > > > > > Anders, bisection is pointing to first bad commit,
> > > > > > > > > 546694b8f658 autofs: add autofs_parse_fd()
> > > > > > > > >
> > > > > > > > > Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> > > > > > > > > Reported-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
> > > > > > > > I tried to find something in that commit that would be different
> > > > > > > > in compat mode, but don't see anything at all -- this appears
> > > > > > > > to be just a simple refactoring of the code, unlike the commits
> > > > > > > > that immediately follow it and that do change the mount
> > > > > > > > interface.
> > > > > > > >
> > > > > > > > Unfortunately this makes it impossible to just revert the commit
> > > > > > > > on top of linux-next. Can you double-check your bisection by
> > > > > > > > testing 546694b8f658 and the commit before it again?
> > > > > > > I tried these two patches again:
> > > > > > > 546694b8f658 ("autofs: add autofs_parse_fd()") - doesn't boot
> > > > > > > bc69fdde0ae1 ("autofs: refactor autofs_prepare_pipe()") - boots
> > > > > > One difference that I notice between those two patches is that we no
> > > > > > long call autofs_prepare_pipe().  We just call autofs_check_pipe().
> > > > > Indeed, so some of the f_flags end up being different. I assumed
> > > > > this was done intentionally, but it might be worth checking if
> > > > > the patch below makes any difference when the flags get put
> > > > > back the way they were. This is probably not the correct fix, but
> > > > > may help figure out what is going on. It should apply to anything
> > > > > from 546694b8f658 ("autofs: add autofs_parse_fd()") to the current
> > > > > linux-next:
> > > > >
> > > > > --- a/fs/autofs/inode.c
> > > > > +++ b/fs/autofs/inode.c
> > > > > @@ -358,6 +358,11 @@ static int autofs_fill_super(struct super_block
> > > > > *s, struct fs_context *fc)
> > > > >           pr_debug("pipe fd = %d, pgrp = %u\n",
> > > > >                    sbi->pipefd, pid_nr(sbi->oz_pgrp));
> > > > >    +        /* We want a packet pipe */
> > > > > +        sbi->pipe->f_flags |= O_DIRECT;
> > > > > +        /* We don't expect -EAGAIN */
> > > > > +        sbi->pipe->f_flags &= ~O_NONBLOCK;
> > > > > +
> > > >
> > > > That makes sense, we do want a packet pipe and that does also mean
> > > >
> > > > we don't want a non-blocking pipe, it will be interesting to see
> > > >
> > > > if that makes a difference. It's been a long time since Linus
> > > >
> > > > implemented that packet pipe and I can't remember now what the
> > > >
> > > > case was that lead to it.
> > > After thinking about this over the weekend I'm pretty sure my mistake
> > >
> > > is dropping the call to autofs_prepare_pipe() without adding the tail
> > >
> > > end of it into autofs_parse_fd().
> > >
> > >
> > > To explain a bit of history which I'll include in the fix description.
> > >
> > > During autofs v5 development I decided to stay with the existing usage
> > >
> > > instead of changing to a packed structure for autofs <=> user space
> > >
> > > communications which turned out to be a mistake on my part.
> > >
> > >
> > > Problems arose and they were fixed by allowing for the 64 bit to 32 bit
> > >
> > > size difference in the automount(8) code.
> > >
> > >
> > > Along the way systemd started to use autofs and eventually encountered
> > >
> > > this problem too. systemd refused to compensate for the length difference
> > >
> > > insisting it be fixed in the kernel. Fortunately Linus implemented the
> > >
> > > packetized pipe which resolved the problem in a straight forward and
> > >
> > > simple way.
> > >
> > >
> > > So I pretty sure that the cause of the problem is the inadvertent
> > > dropping
> > >
> > > of the flags setting in autofs_fill_super() that Arnd spotted although I
> > >
> > > don't think putting it in autofs_fill_super() is the right thing to do.
> > >
> > >
> > > I'll produce a patch today which includes most of this explanation for
> > >
> > > future travelers ...
> > So I have a patch.
> >
> >
> > I'm of two minds whether to try and use the instructions to reproduce this
> >
> > or not because of experiences I have had with other similar testing
> > automation
> >
> > systems that claim to provide a reproducer and end up a huge waste of
> > time and
> >
> > are significantly frustrating.
> >
> >
> > Can someone please perform a test for me once I provide the patch?
> Just tested it, and it passed our tests. Added a tested by flag in your patch.
>
> Thanks for the prompt fix.

That's great to hear Anders, thanks for doing the testing, ;)


Ian